The present invention relates to a scanning optical system applicable to an imaging device such as a color printer and a color copier.
A scanning optical system is generally configured such that a laser beam ON/OFF modulated in accordance with image data is dynamically deflected by a polygonal mirror to scan within a certain angular range, and then converged on a surface to be scanned, which is typically evenly charged, by use of an imaging optical system so that a beam spot moves in a predetermined direction (i.e., a main scanning direction) at a constant speed. As the beam spot moves in the main scanning direction, the surface typically moves in a direction (i.e., an auxiliary scanning direction) perpendicular to the main scanning direction. With this configuration, an electrostatic two-dimensional latent image is formed on the surface to be scanned.
A one-polygon tandem scanning optical system is conventionally known as one type of scanning optical systems for color laser beam printer and the like. The one-polygon tandem scanning optical system is configured such that a plurality of beams corresponding to a plurality of color components are emitted from a plurality of beam emitting points, respectively, and that the plurality of beams are deflected by use of a single polygonal mirror. That is, a plurality of beams are simultaneously incident on the single polygonal mirror, and are directed to a plurality of imaging optical systems corresponding to the color components, respectively. The plurality of beams are then directed onto the surfaces to be scanned, thereby electrostatic latent images corresponding to respective color components are formed on the surfaces.
An exemplary configuration of a scanning optical system, in which a plurality of laser beams are simultaneously deflected by a single mirror, is disclosed in Japanese Patent Provisional Publication No. HEI 10-133131. In this publication, in order to introduce a plurality of laser beams to a plurality of imaging optical systems which are arranged in the auxiliary scanning direction spaced with each other, respectively, a plurality of laser beams are made incident on the polygonal mirror such that the laser beams are parallel with each other and aligned in the auxiliary scanning direction as spaced with each other.
In such a configuration, however, due to a mechanical limitation, a distance between the laser beams cannot be made smaller than a predetermined amount. Therefore, in the conventional one-polygon tandem scanning optical system, a dimension of the polygonal mirror in the auxiliary scanning direction (i.e., a thickness of the polygonal mirror) should be made relatively large. When the polygonal mirror is made larger, a motor to revolve the polygonal mirror is required to have a strong power, which increases a manufacturing cost of the conventional one-polygon tandem scanning optical system.
In order to avoid the above problem, an improved scanning optical system has been suggested. In the improved conventional scanning optical system, a plurality of laser beams corresponding to a plurality of color components are incident on the polygonal mirror as inclined so that the laser beams are incident on substantially the same position.
FIG. 22 schematically shows a configuration of the above-described conventional scanning device.
FIG. 22 shows a side view of the scanning optical system. As shown in FIG. 22, four laser beams are incident on a polygonal mirror 90 at the substantially same point and are directed to an imaging optical system 9a, the four beams being gradually separated as the beams travel from the polygonal mirror 90 to the imaging optical system 9a. The laser beams passed through the imaging optical system 9a are reflected by mirror 900, which are provided for respective beams. The four beams reflected by the mirror 900 are directed to long lenses 9y, 9m, 9c and 9k, which are extending in the main scanning direction, respectively. Then, the four beams are incident on photoconductive drums 9Y, 9M, 9C and 9K, respectively. With this configuration, since a plurality of beams are incident on the same position on the polygonal mirror, the thickness of the polygonal mirror can be made small.
When a plurality of beams are incident on the same point at different incident angles in the auxiliary scanning direction, various aberration occurs for the beams (e.g. bow or curvature of scanning line, wavefront distortion, etc.). It should be noted that degree of the aberrations depends on the incident angle of each beam in the auxiliary scanning direction. Therefore, problems as described below may occur.
In the above-described scanning optical system, in order to compensate for the aberrations due to the inclination of the beams, with respect to the reflection surface of the polygonal mirror, in the auxiliary scanning direction completely by the long lenses 9y, 9m, 9c and 9k, which are provided for respective beams, each of the long lenses 9y, 9m, 9c and 9k should be made to have an intrinsic surface configuration corresponding to the incident angle of the beam in the auxiliary scanning direction (in other words, the degree of aberrations). In the example of FIG. 22, since there are four beams, at least two types of surface configurations are required. When the lenses are formed by molding, a plurality of molds corresponding to the required surface configurations should be prepared, which increases a manufacturing cost of the entire scanning optical system. An example of the scanning optical system as described above is disclosed in Japanese Patent Provisional Publication No. P2003-75751.
However, when the surfaces of the long lenses for the beams are the same as in the above-described scanning optical system, it is impossible to sufficiently compensate for the above-described aberrations with maintaining the fundamental scanning performance required as the scanning optical system. That is, according to the conventional scanning optical system, some aberration is remain insufficiently compensated.